High rate deposition systems and processes for forming hermetic barrier layers

ABSTRACT

A method of forming a hermetic barrier layer comprises sputtering a thin film from a sputtering target, wherein the sputtering target includes a sputtering material such as a low T g  glass, a precursor of a low T g  glass, or an oxide of copper or tin. During the sputtering, the formation of defects in the barrier layer are constrained to within a narrow range and the sputtering material is maintained at a temperature of less than 200° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/840,752, filed on Mar. 15, 2013, which claims the benefit of priorityunder 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/731,226filed on Nov. 29, 2012, the contents of which are relied upon andincorporated herein by reference in their entireties.

BACKGROUND

The present disclosure relates generally to hermetic barrier layers, andmore specifically to sputtering targets and high-throughput physicalvapor deposition methods for forming hermetic barrier layers.

Hermetic barrier layers can be used to protect sensitive materials fromdeleterious exposure to a wide variety of liquids and gases. As usedherein, “hermetic” refers to a state of being completely orsubstantially sealed, especially against the escape or entry of water orair, though protection from exposure to other liquids and gases iscontemplated.

Approaches to creating hermetic barrier layers include physical vapordeposition (PVD) methods such as evaporation or sputtering, and chemicalvapor deposition (CVD) methods such as plasma-enhanced CVD (PECVD).Using such methods, a hermetic barrier layer can be formed directly overthe device or material to be protected. Alternatively, hermetic barrierlayers can be formed on an intermediate structure such as a substrate ora gasket, which can cooperate with an additional structure to provide ahermetically-sealed workpiece.

Both reactive and non-reactive sputtering can be used to form a hermeticbarrier layer, for instance, under room temperature or elevatedtemperature deposition conditions. Reactive sputtering is performed inconjunction with a reactive gas such as oxygen or nitrogen, whichresults in the formation of a corresponding compound barrier layer(i.e., oxide or nitride). Non-reactive sputtering can be performed usingan oxide or nitride target having a desired composition in order to forma barrier layer having a similar or related composition.

On the one hand, reactive sputtering processes typically exhibit fasterdeposition rates than non-reactive processes, and thus may possess aneconomic advantage in certain methods. However, although increasedthroughput can be achieved via reactive sputtering, its inherentlyreactive nature may render such processes incompatible with sensitivedevices or materials that require protection.

Economical sputtering materials, including sputtering targets that canbe used to protect sensitive workpieces such as devices, articles or rawmaterials from undesired exposure to oxygen, water, heat or othercontaminants are highly desirable.

SUMMARY

Disclosed herein are methods for preparing sputtering targets comprisinglow melting temperature (LMT) glass compositions and attendant methodsfor high-rate deposition of thin barrier layers exhibiting aself-passivating attribute. Operating conditions are selected to limitthe surface temperature of the target during sputtering while providingfor the formation of a deposited barrier layer having a small size aswell as a small number density of individual defects such as pinholes.In embodiments, the number density and defect size is constrained to liebelow a critical threshold.

In accordance with various embodiments, self-passivating barrier layerscomprising low melting temperature glass compositions can be formed atrelatively high throughput (high deposition rate) by, inter alia,lowering the sputtering chamber background pressure, preciselycontrolling the substrate and sputtering target temperature, sweepingthe energy source (e.g., plasma) that engages the sputtering target, andlimited the flux of sputtered material to a narrow angle.

A method of forming a hermetic barrier layer comprises providing asubstrate and a sputtering target within a sputtering chamber,maintaining the sputtering material at less than 200° C., and sputteringthe sputtering material with a power source to form a barrier layercomprising the sputtering material over a surface of the substrate. Thesputtering target includes a sputtering material formed over a thermallyconductive backing plate. The sputtering material may include a lowT_(g) glass, a precursor of a low T_(g) glass, or an oxide of copper ortin.

The power source may include an ion source, a laser, plasma, a magnetronor combinations thereof. For example, the sputtering may comprise ionbeam-assisted deposition. A further example may include a remote plasmageneration sputter system that features independent (non-coupled)control of ion generation and density at the source, as well as controlof the ion current, and voltage biasing to the target.

During formation of the barrier layer, the power source may betranslated with respect to the sputtering target and/or the sputteringtarget may be rotated. In addition to maintaining the sputteringmaterial at less than 200° C., the substrate may be maintained at lessthan 200° C. Hermetic barrier layers can be formed using the disclosedprocess at a deposition rate of at least 10 A/sec.

Sputter conditions are chosen to ensure the defect size and densitydistributions in the deposited layer are sufficiently small that defectdiffusion paths can be effectively sealed upon exposure to moisture oroxygen. Sealing proceeds by virtue of the deposited layer'sself-passivating attribute. We have shown that inorganic oxidesexhibiting a molar volume expansion of from 1% to 15% upon reaction withwater or oxygen are candidates for hermetic barrier layer formation.

Passivation may occur “passively” by simple exposure to ambientconditions, or “actively” by submerging the barrier layer in a waterbath or exposing it to steam. The average defect size and density may beless than the expansion associated with the molar volume expansion ofthe as-deposited layer that accompanies passivation. Depositionconditions are used to ensure the defect size and density distributionswithin the barrier layer result in a population of void spaces that canbe sealed from the molar volume expansion.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theinvention as described herein, including the detailed description whichfollows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate various embodiments of theinvention and together with the description serve to explain theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of magnetron sputter deposition rate versus power;

FIG. 2 is a plot of magnetron sputter deposition rate and barrier layeruniformity versus substrate-target distance;

FIG. 3 is a schematic diagram of a single chamber sputter tool forforming hermetic barrier layers;

FIG. 4 is a Thornton diagram depicting various barrier layermicrostructures;

FIG. 5 is a plot of barrier layer composition versus deposition rate fordifferent magnetron sputtering conditions;

FIG. 6 is a plot of Sn⁴⁺ content versus deposition rate for aniobium-doped tin fluorophosphate glass material;

FIG. 7 is an illustration of a hermetic barrier layer formed over asurface of a substrate;

FIG. 8 depicts a portion of an RF sputtering apparatus according to anexample embodiment;

FIG. 9 depicts a portion of a continuous in-line magnetron sputteringapparatus according to a further example embodiment;

FIG. 10 in an illustration of a calcium-patch test sample foraccelerated evaluation of hermeticity;

FIGS. 11A-11F show test results for non-hermetically sealed (left, FIGS.11A-11C) and hermetically sealed (right, FIGS. 11D-11F) calcium patchesfollowing accelerated testing;

FIGS. 12A-12D show glancing angle (FIG. 12A, FIG. 12C) and thin film(FIG. 12B, FIG. 12D) x-ray diffraction (XRD) spectra for a hermeticCuO-based barrier layer-forming material (top series) and a non-hermeticCu₂O-based barrier layer forming material (bottom series);

FIGS. 13A-13I show a series of glancing angle XRD spectra for hermeticCuO-based barrier layers following accelerated testing;

FIGS. 14A-14B are series of glancing angle XRD spectra for hermeticSnO-based barrier layers (FIG. 14A) and non-hermetic SnO₂-based barrierlayers (FIG. 14B) following accelerated testing;

FIG. 15 is a photograph of a copper backing plate according to variousembodiments;

FIG. 16 is a photograph of a solder-coated copper backing plate;

FIG. 17 is an image of an example sputtering target comprising anannealed low T_(g) glass material;

FIG. 18 in an image of a pressed low T_(g) glass sputtering target;

FIG. 19 shows a large form factor sputtering target prior tocompressing;

FIG. 20 shows a circular copper backing plate with loose powder materialincorporated into a central area of the plate; and

FIG. 21 shows the circular copper backing plate of FIG. 20 aftercompression of the loose powder.

DETAILED DESCRIPTION

Mechanically-stable hermetic barrier layers can be formed by physicalvapor deposition (e.g., sputter deposition or laser ablation) of asuitable starting material directly onto a workpiece or onto a substratethat can be used to encapsulate a workpiece. The starting materialsinclude low T_(g) glass materials and their precursors, as well aspolycrystalline or amorphous oxides of copper or tin. As defined herein,a low T_(g) glass material has a glass transition temperature of lessthan 400° C., e.g., less than 350, 300, 250 or 200° C.

In embodiments, the number and size distribution of defects that may beformed within the hermetic barrier layers are constrained to a narrowspecified range. By limiting the population of defects, the as-formedlayers through a self-passivation mechanism can effectively compensatefor such defects during exposure to air or moisture and form a hermeticlayer.

Example processes for forming hermetic barrier layers include ion beamsputtering, magnetron sputtering, laser ablation, remote plasmageneration high target utilization sputtering (HiTUS) and ionbeam-assisted deposition (IBAD). Ion beam-assisted deposition is acombination of two distinct physical operations, physical vapordeposition of a target material onto a substrate and simultaneousbombardment of the substrate surface with an ion beam. Each of theforegoing approaches can be implemented in a batch or continuous, e.g.,roll-to-roll process. With each of these techniques, the deposition rateof the barrier layer (low melting temperature glass) material can beincreased by increasing the energy or flux of the corresponding ion orphoton source. However, such a pedestrian approach does not ensure thatthe defect population within the formed layer will enable successfulformation of a hermetic barrier layer.

Embodiments relate to high-rate (e.g., greater than 10 A/sec) depositionof the hermetic barrier layers. The deposition rate can be, for example,at least 10, 20, 50 or 100 A/sec. High-rate deposition of hermeticbarrier layers can be successfully carried out by limiting thetemperature of the target during deposition, e.g., to less than 200° C.,and by constraining the defect number and size distribution contributingto the barrier layer's void space to be lower than what can be sealedfrom the molar volume expansion accompanying passivation.

According to embodiments, hermetic barrier layers can be formed usingion beam deposition processes. The ion beam-derived layers can exhibitdefect densities on the order of 2×10⁻²/cm², which can be up to fiveorders of magnitude less than the defect densities associated with mostmagnetron sputtering approaches. Beneficial aspects of the ion beamdeposition approach, particularly in comparison to magnetron sputtering,include a more directional flux (e.g., near normal incidence), lowerchamber background pressure, higher mean free path, and the ability toindependently adjust the ion flux energy and power. Near-normal ion beamsputtering conditions have been demonstrated to reduce the size ofdefects up to an order of magnitude over off-normal sputteringconditions.

In the case of magnetron sputtering, a dominant source of particlecontamination is related to the finite cross-section of the sputteringtarget surface that is exposed to weaker plasma density. Thisnon-homogeneous plasma density can cause the formation, migration andmechanical ejection of filaments or nodules (defects) from the targetduring deposition. Such a defect-formation mechanism is undesirablyexacerbated by applying higher ion sputtering gas densities.

In embodiments, magnetron sputtering can be used to form high depositionrate, high throughput, low defect density hermetic barrier layers by,for instance, limiting the angular components of the deposition process,for example through baffling, and/or by using grounding grids in thedeposition chamber.

In a similar vein, in the case of laser ablation, the ejection ofparticulates with significant size occurs when the photon flux is highenough to induce explosive ablation. When this occurs, a highly-directedforward jet of target material accompanies a broad-angle plume. Theplume typically contains many fine particles, from ˜0.01 μm to 10 μm,which are ejected directly from the target surface.

In embodiments, the hermetic barrier layers are formed by anon-equilibrium deposition process and have a defect size anddistribution where the volume in the layer occupied by as-depositeddefects is less than 15% of the total film volume afterself-passivation.

In conjunction with the instant methods, also disclosed are approachesfor preparing sputtering targets and associated processing conditionsfor forming hermetic barrier layers from a class of low meltingtemperature (i.e., low T_(g)) glass compositions. The barrier layersexhibit a self-passivating attribute that results in layer that ishermetic to water and oxygen.

Modeled data indicate that high-rate deposition can be achieved bymaintaining the sputtering target surface temperature below 200° C.,e.g., below 180° C. or 160° C. during deposition. The target temperaturecan be controlled, for example, by sweeping processing plasma over alarge area of the target surface. Swept plasma limits the thermal loadto any one given target location despite a higher applied power and/orhigher ion flux, which can be used to accelerate the deposition rate.Swept plasma may also result in better target utilization, with more ofthe target material consumed in film formation. In contrast to thedisclosed approach, sputtering target surface temperatures that exceed200° C. usually accompany a catastrophic failure of the target, e.g.,fracture of the target material, delamination from the backing plateand/or evidence of significant chemical attack.

Sweeping the plasma over the target surface can be performed whileactively cooling the target using, for example, a moving magnet,rotating cylinder, or similar design. Ion beam flux designs maysimilarly employ sweeping beams over cooled glass targets. During thesputtering process, which is carried out within a vacuum chamber, aninternal pressure of the chamber can be less than 10⁻³ Torr, e.g., lessthan 1×10⁻³, 5×10⁻⁴ or 1×10⁻⁴ Torr.

FIG. 1 is a plot of magnetron sputter deposition rate versus power for asputtering target comprising a low T_(g) tin fluorophosphate glass. Thedata show results from swept plasma (curve A) over a cooled (180-200°C.) sputter target as well as for a static, immobile plasma (curvesB-E).

FIG. 2 is a plot of magnetron sputter deposition rate and barrier layeruniformity versus distance from the substrate to the target for a sweptplasma over a cooled (180-200° C.) sputter target at 140 W.

Suitable deposition methods include non-equilibrium processes such asion beam sputtering, magnetron sputtering, and laser ablation. Suchnon-equilibrium processes can be used to ensure sufficient volumetricswelling of the barrier layer material, but limit the expansion to lessthan 15%, e.g., less than 10%. By limiting the number and sizedistribution of defects within the barrier layer to a narrow range, thevolumetric expansion of the material that occurs through reaction withmoisture can effectively pinch off pores and other defects to form aself-passivated, hermetic barrier layer.

A variety of deposition apparatus can be used to form the hermeticbarrier layers. In accordance with an example embodiment, asingle-chamber sputter deposition apparatus 100 for forming such barrierlayers is illustrated schematically in FIG. 3. While the apparatus andattendant methods are described below with respect to deposition onto asubstrate, it will be appreciated that the substrate may be replaced bya workpiece or other device that is to be protected by the barrierlayer.

Apparatus 100 includes a vacuum chamber 105 having a substrate stage 110onto which one or more substrates 112 can be mounted, and a mask stage120, which can be used to mount shadow masks 122 for patterneddeposition of different layers onto the substrates. The chamber 105 isequipped with a vacuum port 140 for controlling the interior pressure,as well as a water cooling port 150 and a gas inlet port 160. The vacuumchamber can be cryo-pumped (CTI-8200/Helix; MA, USA) and is capable ofoperating at pressures suitable for both evaporation processes (˜10⁻⁶Torr) and RF sputter deposition processes (˜10⁻³ Torr).

As shown in FIG. 3, multiple evaporation fixtures 180, each having anoptional corresponding shadow mask 122 for evaporating material onto asubstrate 112, are connected via conductive leads 182 to a respectivepower supply 190. A target material 200 to be evaporated can be placedinto each fixture 180. Thickness monitors 186 can be integrated into afeedback control loop including a controller 193 and a control station195 in order to affect control of the amount of material deposited.

In an example system, each of the evaporation fixtures 180 are outfittedwith a pair of copper leads 182 to provide DC current at an operationalpower of about 80-180 Watts. The effective fixture resistance willgenerally be a function of its geometry, which will determine theprecise current and wattage.

An RF sputter gun 300 having a sputtering target 310 is also providedfor forming a barrier layer on a substrate. The RF sputter gun 300 isconnected to a control station 395 via an RF power supply 390 andfeedback controller 393. For sputtering inorganic, hermetic layers,water-cooled cylindrical RF sputtering guns (Onyx-3™, Onyx-R™, AngstromSciences, PA) can be positioned within the chamber 105. Suitable RFdeposition conditions include 50-150 W forward power (<1 W reflectedpower), which corresponds to a typical deposition rate of about ˜5Å/second (Advanced Energy, Co, USA).

A post-deposition sintering or annealing step of the as-depositedmaterial may be performed or omitted. An optional annealing step canreduce internal stresses within the barrier layer.

In general, suitable materials for forming hermetic barrier layersinclude low T_(g) glasses and suitably reactive oxides of copper or tin.Hermetic barrier layers can be formed from low T_(g) materials such asphosphate glasses, borate glasses, tellurite glasses and chalcogenideglasses. Example borate and phosphate glasses include tin phosphates,tin fluorophosphates and tin fluoroborates. Sputtering targets caninclude such glass materials or, alternatively, precursors thereof.Example copper and tin oxides are CuO and SnO, which can be formed fromsputtering targets comprising pressed powders of these materials.

Optionally, the compositions can include one or more dopants, includingbut not limited to tungsten, cerium and niobium. Such dopants, ifincluded, can affect, for example, the optical properties of the barrierlayer, and can be used to control the absorption by the barrier materialof electromagnetic radiation, including laser radiation. For instance,doping with ceria can increase the absorption by a low T_(g) glassbarrier at laser processing wavelengths, which can enable the use oflaser-based sealing techniques after formation on a substrate or gasket.

Example tin fluorophosphate glass compositions can be expressed in termsof the respective compositions of SnO, SnF₂ and P₂O₅ in a correspondingternary phase diagram. Suitable tin fluorophosphates glasses include20-100 mol % SnO, 0-50 mol % SnF₂ and 0-30 mol % P₂O₅. These tinfluorophosphates glass compositions can optionally include 0-10 mol %WO₃, 0-10 mol % CeO₂ and/or 0-5 mol % Nb₂O₅.

For example, a composition of a doped tin fluorophosphate startingmaterial suitable for forming a hermetic barrier layer comprises 35 to50 mole percent SnO, 30 to 40 mole percent SnF₂, 15 to 25 mole percentP₂O₅, and 1.5 to 3 mole percent of a dopant oxide such as WO₃, CeO₂and/or Nb₂O₅.

A tin fluorophosphate glass composition according to one particularembodiment is a niobium-doped tin oxide/tin fluorophosphate/phosphoruspentoxide glass comprising about 38.7 mol % SnO, 39.6 mol % SnF₂, 19.9mol % P₂O₅ and 1.8 mol % Nb₂O₅. Sputtering targets that can be used toform such a glass layer may include, expressed in terms of atomic molepercent, 23.04% Sn, 15.36% F, 12.16% P, 48.38% O and 1.06% Nb.

A tin phosphate glass composition according to an alternate embodimentcomprises about 27% Sn, 13% P and 60% O, which can be derived from asputtering target comprising, in atomic mole percent, about 27% Sn, 13%P and 60% O. As will be appreciated, the various glass compositionsdisclosed herein may refer to the composition of the deposited layer orto the composition of the source sputtering target.

As with the tin fluorophosphates glass compositions, example tinfluoroborate glass compositions can be expressed in terms of therespective ternary phase diagram compositions of SnO, SnF₂ and B₂O₃.Suitable tin fluoroborate glass compositions include 20-100 mol % SnO,0-50 mol % SnF₂ and 0-30 mol % B₂O₃. These tin fluoroborate glasscompositions can optionally include 0-10 mol % WO₃, 0-10 mol % CeO₂and/or 0-5 mol % Nb₂O₅.

Typical prescriptions for managing thin film structure, including thenumber and size of defects associated with thin film deposition, areillustrated with a Thornton diagram (see FIG. 4). The Thornton diagramshows deposited thin film morphological regions arising from differentsputtering gas pressure and substrate temperature conditions, wheremicrostructure is segmented into zone I, zone T, zone II, and zone IIImorphologies. These zones arise from different sputtering gas pressureand substrate temperature conditions. Zone I films (low T_(S), highP_(G)) typically exhibit a microstructure of columnar crystallites 402with voids in between the columns while zone II (high T_(S)) exhibits amicrostructure of columnar grains 404 separated by distinct denseinter-crystalline boundaries. Zone T (low T_(S), low P_(G)) is atransition zone in between zone I and II consisting of a poorly-defineddense array of fibrous grains 406 without voided boundaries. Arecrystallized grain structure 408 is illustrated in Zone III.

Due to their relatively low melting temperature and chemical liability,process conditions and the resulting layers that include the glasscompositions disclosed herein exhibit significant deviation from typicalrefractory materials. For instance, applicants have shown that theself-passivating character of tin-containing glass compositions can becorrelated to the Sn²⁺ (i.e., SnO) content within the formed layer. Datashow that the Sn²⁺ content is a function of the substrate temperature,and that Sn²⁺ rich layers can be formed by cooling the substrate duringdeposition. At higher substrate temperatures, lower amounts of Sn²⁺ areincorporated into the barrier layer due to the loss of PO_(x)F_(y) andSnF_(x) species at the expense of Sn⁴⁺ (i.e., SnO₂). Thin film layersthat incorporate a large fraction of Sn⁴⁺ do not readily self-passivateand therefore do not form effective barrier layers.

During formation of the barrier layer, the substrate can be maintainedat a temperature less than 200° C., e.g., less than 200, 150, 100, 50 or23° C. In embodiments, the substrate is cooled to a temperature lessthan room temperature during deposition of the barrier layer. The targettemperature as well as the substrate temperature can be controlled inboth ion-beam deposition processes and magnetron sputter depositionprocesses.

FIG. 5 is a plot of barrier layer composition (wt. %) versus depositionrate (A/sec) for increasing values of magnetron sputter power (50, 70,90 or 110 Watts). The initial sputtering target composition included49.2 wt. % oxygen, 23.0 wt. % tin, 14.5 wt. % fluorine, 12.3 wt. %phosphorus and 1.0 wt. % Nb. The filled data points correspond to asubstrate temperature of 45° C., while the open data points correspondto a substrate temperature of 15° C. The sputtering target temperaturewas maintained less than 200° C.

FIG. 6 is a corresponding plot of percentage Sn⁴⁺ (i.e., Sn⁴⁺/total Sncontent) versus deposition rate. As with FIG. 5, the filled data pointscorrespond to a substrate temperature of 45° C., while the open datapoints correspond to a substrate temperature of 15° C. The FIG. 6 dataclearly show that the Sn⁴⁺ content in the barrier layers can beadvantageously suppressed by cooling the substrate.

Additional aspects of suitable low T_(g) glass compositions and methodsused to form glass layers from these materials are disclosed incommonly-assigned U.S. Pat. No. 5,089,446 and U.S. patent applicationSer. Nos. 11/207,691, 11/544,262, 11/820,855, 12/072,784, 12/362,063,12/763,541 and 12/879,578, the entire contents of which are incorporatedby reference herein.

The hermetic barrier layer materials disclosed herein may comprise abinary, ternary or higher-order composition. A survey of several binaryoxide systems reveals other materials capable of formingself-passivating hermetic barrier layers. In the copper oxide system,for example, as-deposited amorphous CuO reacts with moisture/oxygen topartially form crystalline Cu₄O₃ and the resulting composite layerexhibits good hermeticity. When Cu₂O is deposited as the first inorganiclayer, however, the resulting film is not hermetic. In the tin oxidesystem, as-deposited amorphous SnO reacts with moisture/oxygen topartially form crystalline Sn₆O₄(OH)₄ and SnO₂. The resulting compositelayer exhibits good hermeticity. When SnO₂ is deposited as the firstinorganic layer, however, the resulting film is not hermetic.

According to various sputtering approaches, a self-passivating layer canbe formed on a surface of a substrate or workpiece from a suitabletarget material. The self-passivating layer is an inorganic material.Without wishing to be bound by theory, it is believed that, according tovarious embodiments, during or after its formation, the as-depositedlayer reacts with moisture or oxygen to form a mechanically-stablehermetic barrier layer. The hermetic barrier layer comprises theas-deposited layer and a second inorganic layer, which is the reactionproduct of the deposited layer with moisture or oxygen. Thus, the secondinorganic layer forms at the ambient interface of the as-depositedlayer. A schematic of a hermetic barrier layer 704 formed over a surfaceof a substrate 700 is illustrated in FIG. 7. In the illustratedembodiment, the hermetic barrier layer 704 comprises a first(as-deposited) inorganic layer 704A, and a second (reaction product)inorganic layer 704B. In embodiments, the first and second layers cancooperate to form a composite thin film that can isolate and protect anunderlying structure. The passivatable as-deposited layer comprises alow T_(g) glass material or an oxide of copper or tin

According to further embodiments, a molar volume of the passivatedsecond inorganic layer material is from about 1% to 15% greater than amolar volume of the first inorganic layer material, and an equilibriumthickness of the second inorganic layer is at least 10% of but less thanan initial thickness of the first inorganic layer. While the firstinorganic layer can be amorphous, the second inorganic layer can be atleast partially crystalline.

In embodiments, the molar volume change (e.g., increase) manifests as acompressive force within the composite barrier layer that contributes toa self-sealing phenomenon. Because the second layer is formed as thespontaneous reaction product of the first inorganic layer with oxygen orwater, as-deposited layers (first inorganic layers) that successfullyform hermetic barrier layers are less thermodynamically stable thantheir corresponding second inorganic layers. Thermodynamic stability isreflected in the respective Gibbs free energies of formation.

The hermetic barrier layers disclosed herein may be characterized asthin film materials. A total thickness of a hermetic barrier layer canrange from about 150 nm to 200 microns. In various embodiments, athickness of the as-deposited layer can be less than 200 microns, e.g.,less than 200, 100, 50, 20, 10, 5, 2, 1, 0.5 or 0.2 microns. Examplethicknesses of as-deposited glass layers include 200, 100, 50, 20, 10,5, 2, 1, 0.5, 0.2 or 0.15 microns.

Hermetic barrier layers formed by physical vapor deposition according tothe present disclosure may exhibit a self-passivating attribute thatefficiently and significantly impedes moisture and oxygen diffusion.

According to embodiments, the choice of the hermetic barrier layermaterial(s) and the processing conditions for forming hermetic barrierlayers over a workpiece or substrate are sufficiently flexible that theworkpiece or substrate is not adversely affected by formation of thebarrier layer.

Example sputtering configurations according to various embodiments areillustrated in FIGS. 8 and 9. FIG. 8 shows RF sputtering from asputtering target 310 to form a barrier layer on a substrate 112 that issupported by a rotating substrate stage 110 as also depicted in FIG. 3.FIG. 9 shows a portion of an in-line planar magnetron sputteringapparatus configured to continuously form a hermetic barrier layer on asurface of a translating substrate. A direction of motion of thesubstrate is shown in FIG. 9 by arrow A. The pristine substrate can beunwrapped from a first roll, passed over a deposition zone of themagnetron sputtering target 311 to provide a barrier layer on a portionof the workpiece, and then the coated workpiece can be wrapped onto asecond roll.

A hermetic layer is a layer which, for practical purposes, is consideredsubstantially airtight and substantially impervious to moisture and/oroxygen. By way of example, the hermetic thin film can be configured tolimit the transpiration (diffusion) of oxygen to less than about 10⁻²cm³/m²/day (e.g., less than about 10⁻³ cm³/m²/day), and limit thetranspiration (diffusion) of water to about 10⁻² g/m²/day (e.g., lessthan about 10⁻³, 10⁻⁴, 10⁻⁵ or 10⁻⁶ g/m²/day). In embodiments, thehermetic thin film substantially inhibits air and water from contactingan underlying workpiece or a workpiece sealed within a structure usingthe hermetic material.

To evaluate the hermeticity of the hermetic barrier layers, calciumpatch test samples were prepared using the single-chamber sputterdeposition apparatus 100. In a first step, calcium shot (Stock # 10127;Alfa Aesar) was evaporated through a shadow mask 122 to form 25 calciumdots (0.25 inch diameter, 100 nm thick) distributed in a 5×5 array on a2.5 inch square glass substrate. For calcium evaporation, the chamberpressure was reduced to about 10⁻⁶ Torr. During an initial pre-soakstep, power to the evaporation fixtures 180 was controlled at about 20 Wfor approximately 10 minutes, followed by a deposition step where thepower was increased to 80-125 W to deposit about 100 nm thick calciumpatterns on each substrate.

Following evaporation of the calcium, the patterned calcium patches wereencapsulated using comparative inorganic oxide materials as well ashermetic inorganic oxide materials according to various embodiments. Theinorganic oxide materials were deposited using room temperature RFsputtering of pressed powder or glass sputtering targets. The pressedpowder targets were prepared separately using a manual heated bench-tophydraulic press (Carver Press, Model 4386, Wabash, Ind., USA). The presswas typically operated at 5,000 psi for 2 hours at about 200° C.

The RF power supply 390 and feedback control 393 (Advanced Energy, Co,USA) were used to form first inorganic oxide layers over the calciumhaving a thickness of about 2 micrometers. No post-deposition heattreatment was used. Chamber pressure during RF sputtering was about 1milliTorr. The formation of a second inorganic layer over the firstinorganic layer was initiated by ambient exposure of the test samples toroom temperature and atmospheric pressure prior to testing.

FIG. 10 is a cross-sectional view of a test sample comprising a glasssubstrate 900, a patterned calcium patch (˜100 nm) 902, and an inorganicoxide film (˜2 μm) 904. Following ambient exposure, the inorganic oxidefilm 904 comprises a first inorganic layer 904A and a second inorganiclayer 904B. In the illustrated embodiment, the second inorganic layer isformed over a major surface of the first inorganic layer. In anon-illustrated embodiment, the second inorganic layer may also beformed over the exposed edges (side surfaces) of the first inorganiclayer. In order to evaluate the hermeticity of the inorganic oxide film,calcium patch test samples were placed into an oven and subjected toaccelerated environmental aging at a fixed temperature and humidity,typically 85° C. and 85% relative humidity (“85/85 testing”).

The hermeticity test optically monitors the appearance of thevacuum-deposited calcium layers. As-deposited, each calcium patch has ahighly reflective metallic appearance. Upon exposure to water and/oroxygen, the calcium reacts and the reaction product is opaque, white andflaky. Survival of the calcium patch in the 85/85 oven over 1000 hoursis equivalent to the encapsulated film surviving 5-10 years of ambientoperation. The detection limit of the test is approximately 10⁻⁷ g/m²per day at 60° C. and 90% relative humidity.

FIG. 11 illustrates behavior typical of non-hermetically sealed andhermetically sealed calcium patches after exposure to the 85/85accelerated aging test. In FIG. 11, the left column shows non-hermeticencapsulation behavior for Cu₂O films formed directly over the patches.All of the Cu₂O-coated samples failed the accelerated testing, withcatastrophic delamination of the calcium dot patches evidencing moisturepenetration through the Cu₂O layer. The right column shows positive testresults for nearly 50% of the samples comprising a CuO-depositedhermetic layer. In the right column of samples, the metallic finish of34 intact calcium dots (out of 75 test samples) is evident.

Both glancing angle x-ray diffraction (GIXRD) and traditional powderx-ray diffraction were used to evaluate the near surface and entireoxide layer, respectively, for both non-hermetic and hermetic depositedlayers. FIG. 12 shows GIXRD data (plots A and C) and traditional powderreflections (plots B and D) for both hermetic CuO-deposited layers(plots A and B) and non-hermetic Cu₂O-deposited layers (plots C and D).Typically, the 1 degree glancing angle used to generate the GIXRD scansof FIGS. 12A and 4C probes a near-surface depth of approximately 50-300nanometers.

Referring still to FIG. 12, the hermetic CuO-deposited film (plot A)exhibits near surface reflections that index to the phase paramelaconite(Cu₄O₃), though the interior of the deposited film (plot B) exhibitsreflections consistent with a significant amorphous copper oxidecontent. The paramelaconite layer corresponds to the second inorganiclayer, which formed from the first inorganic layer (CuO) that was formeddirectly over the calcium patches. In contrast, the non-hermeticCu₂O-deposited layer exhibits x-ray reflections in both scans consistentwith Cu₂O.

The XRD results suggest that hermetic films exhibit a significant andcooperative reaction of the sputtered (as-deposited) material withheated moisture in the near surface region only, while non-hermeticfilms react with heated moisture in their entirety yielding significantdiffusion channels which preclude effective hermeticity. For the copperoxide system, the hermetic film data (deposited CuO) suggest thatparamelaconite crystallite layer forms atop an amorphous base ofun-reacted sputtered CuO, thus forming a mechanically stable andhermetic composite layer.

FIGS. 13A-13H show a series of GIXRD plots, and FIG. 13I shows a BraggXRD spectrum for a CuO-deposited hermetic barrier layers followingaccelerated testing. Bragg diffraction from the entire film volume hasan amorphous character, with the paramelaconite phase present at/nearthe film's surface. Using a CuO density of 6.31 g/cm³, a massattenuation coefficient of 44.65 cm²/g, and an attenuation coefficientof 281.761 cm⁻¹, the paramelaconite depth was estimated from the GIXRDplots of FIG. 13. In FIGS. 13A-13H, successive glancing incident x-raydiffraction spectra obtained at respective incident angles of 1°, 1.5°,2°, 2.5°, 3.0°, 3.5°, 4°, and 4.5° show a surface layer (paramelaconite)that comprises between 31% (619 nm) and 46% (929 nm) of the original 2microns of sputtered CuO after exposure to 85° C. and 85% relativehumidity for 1092 hours. A summary of the calculated surface depth(probed depth) for each GIXRD angle is shown in Table 1.

TABLE 1 Paramelaconite depth profile FIG. GIXRD angle (degrees) ProbedDepth (nm) 13A 1 300 13B 1.5 465 13C 2 619 13D 2.5 774 13E 3 929 13F 3.51083 13G 4 1238 13H 4.5 1392 13I n/a 2000

In addition to the hermeticity evaluations conducted using copperoxide-based barrier layers, tin oxide-based barrier layers were alsoevaluated. As seen with reference to FIG. 14, which shows GIXRD spectrafor SnO (top) and SnO₂-deposited films (bottom) after 85/85 exposure,the hermetic thin film (top) exhibits a crystalline SnO₂-like(passivation) layer that has formed over the deposited amorphous SnOlayer, while the non-hermetic (SnO₂-deposited) film exhibits an entirelycrystalline morphology.

Table 2 highlights the impact of volume change about the central metalion on the contribution to film stress of the surface hydrationproducts. It has been discovered that a narrow band corresponding to anapproximate 15% or less increase in the molar volume change contributesto a hermetically-effective compressive force. In embodiments, a molarvolume of the second inorganic layer is from about -1% to 15% (i.e., −1,0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15%) greater than amolar volume of the first inorganic layer. The resulting self-sealingbehavior (i.e., hermeticity) appears related to the volume expansion.

TABLE 2 Calculated Molar Volume Change for Various Materials SputteringTarget Δ Molar Material/First Second Volume Hermetic Inorganic LayerInorganic Layer [%] Layer? SnO SnO₂ 5.34 yes FeO Fe₂O₃ ^(†) 27.01 noSb₂O₃ Sb₂O₅ ^(†) 63.10 no (senarmonitite) Sb₂O₃ Sb₂O₅ ^(†) 67.05 no(valentinite) Sb₂O₃ Sb + 3Sb + 5O₄ −9.61 no (valentinite) (cervantite)Sb₂O₃ Sb₃O₆(OH) −14.80 no (valentinite) (stibiconite) ^(†) Ti₂O₃ TiO₂^(†) 17.76 no Cu₂O Cu⁺ ₂Cu²⁺ ₂O₃ 12.30 no (paramelaconite) ^(†) CuO Cu⁺₂Cu²⁺ ₂O₃ 0.97 yes (paramelaconite) ^(†) estimate

Table 3 shows the hermetic-film-forming inorganic oxide was always theleast thermodynamically stable oxide, as reflected in its Gibbs freeenergy of formation, for a given elemental pair. This suggests thatas-deposited inorganic oxide films are metastable and thus potentiallyreactive towards hydrolysis and/or oxidation.

TABLE 3 Gibbs Formation Free Energy (ΔG°_(formation)) of Various OxidesTarget Material ΔG°_(formation) [kJ/mol] Hermetic Layer SnO −251.9 yesSnO₂ −515.8 no CuO −129.7 yes Cu₂O −146.0 no

In embodiments, the barrier layer can be derived from room temperaturesputtering of one or more of the foregoing materials, though other thinfilm deposition techniques can be used. In order to accommodate variousworkpiece architectures, deposition masks can be used to produce asuitably patterned hermetic barrier layer. Alternatively, conventionallithography and etching techniques can be used to form a patternedhermetic layer from a previously-deposited blanket layer.

To form hermetic barrier layers via sputtering, a sputtering target maycomprise a low T_(g) glass material or a precursor thereof, such as apressed powder target where the powder constituents have an overallcomposition corresponding to the desired barrier layer composition.Glass-based sputtering targets may comprise a dense, single phase lowT_(g) glass material. Aspects of forming both glass compositionsputtering targets and pressed powder sputtering targets are disclosedherein.

For both glass composition and pressed powder composition targets, athermally-conductive backing plate such as a copper backing plate may beused to support the target material. The backing plate can have anysuitable size and shape. In one example embodiment, a 3 inch outerdiameter (OD) circular copper backing plate is formed from a 0.25 inchthick copper plate. A central area having a diameter of about 2.875 inchis milled from the plate to a depth of about ⅛ inch, leaving anapproximately 1/16 inch wide lip around a peripheral edge of the centralarea. A photograph of such a copper backing plate is shown in FIG. 15.

To form a glass composition sputtering target according to oneembodiment, the central area of the backing plate is initially coatedwith a thin layer of flux-less solder (Cerasolzer ECO-155). The solderprovides an oxide-free, or substantially oxide-free, adhesion-promotinglayer to which the target material can be bonded. An image of asolder-treated copper backing plate is shown in FIG. 16.

A desired glass composition can be prepared from raw starting materials.Starting materials to form a tin fluorophosphate glass, for example, canbe mixed and melted to homogenize the glass. The raw materials, whichcan comprise powder materials, can be heated, for example, in a carboncrucible to a temperature in the range of 500-550° C., and then castonto a graphite block to form a glass cullet. The cullet can be brokenup, remelted (500-550° C.), and then poured into the central area of apre-heated, solder-treated backing plate. The backing plate can bepre-heated to a temperature in the range of 100-125° C. The casting canbe annealed at a temperature of 100-125° C. for 1 hour, though longeranneal times can be used for larger backing plates. An image of anas-annealed low T_(g) glass sputtering target is shown in FIG. 17.

After the glass composition is annealed, the glass can be heat-pressedagainst the solder-coated copper, e.g., using a Carver press at atemperature of below 225° C., e.g., from 140-225° C. and an appliedpressure of 2000-25,000 psi. The heat-pressing promotes thoroughcompaction and good adhesion of the glass material to the backing plate.In a further embodiment, the step of heat-pressing can be performed at atemperature of less than 180° C. An image of a pressed, low T_(g) glasssputtering target is shown in FIG. 18.

By controlling the temperature and pressure used to anneal and compressthe glass target, the formation of unwanted voids or secondary phasescan be minimized or avoided. In accordance with various embodiments, asputtering target comprising a low T_(g) glass material can have adensity approaching or equal to the theoretical density of the glassmaterial. Example target materials include glass material having adensity greater than 95% of a theoretical density of the material (e.g.,at least 96, 97, 98, or 99% dense).

By providing dense sputtering targets, degradation of the target duringuse can be minimized. For instance, the exposed surface of a target thatcontains porosity or mixed phases may become preferentially sputteredand roughened during use as the porosity or second phase is exposed.This can result in a runaway degradation of the target surface. Aroughened target surface may lead to flaking of particulate materialfrom the target, which can lead to the incorporation of defects orparticle occlusions in the deposited layer. A barrier layer comprisingsuch defects may be susceptible to hermetic breakdown. Dense sputteringtargets may also exhibit uniform thermal conductivity, which promotesnon-destructive heating and cooling of the target material duringoperation.

According to various embodiments, methods for forming a sputteringtarget disclosed herein can be used to produce single phase, highdensity targets of a low T_(g) glass composition. The glass targets canbe free of secondary or impurity phases. While the foregoing relates toforming a sputtering target directly on a backing plate, it will beappreciated that a suitable glass-based target composition can beprepared independently from such a backing plate and then optionallyincorporated onto a backing plate in a subsequent step.

In embodiments, a method of making a sputtering target comprising a lowT_(g) glass material comprises providing a mixture of raw materialpowders, heating the powder mixture to form a molten glass, cooling theglass to form a cullet, melting the cullet to form a glass melt, andshaping the glass melt into a solid sputtering target. FIG. 19 is animage showing the incorporation of glass material into the central areaof larger form factor rectangular backing plate.

As an alternative to a glass material-based sputtering target, the stepsof melting and homogenizing the starting raw materials can be omitted,and instead powder raw materials can be mixed and pressed directly intothe central area of a suitable backing plate. FIG. 20 is an imageshowing the incorporation of powder raw materials into the central areaof a circular backing plate, and FIG. 21 shows a final pressed-powdersputtering target after compression of the powder materials of FIG. 20.

A method of making a pressed-powder sputtering target comprising apowder compact having the composition of a low T_(g) glass comprisesproviding a mixture of raw material powders, and pressing the mixtureinto a solid sputtering target. In such an approach, the powder mixtureis a precursor of a low T_(g) glass material. In a related approach, amethod of making a pressed-powder sputtering target comprising an oxideof copper or tin comprises providing a powder of CuO or SnO and pressingthe powder into a solid sputtering target.

Hermetic barrier layers formed by sputtering may be opticallytransparent, which make them suitable for encapsulating, for example,food items, medical devices, and pharmaceutical materials, where theability to view the package contents without opening the package may beadvantageous. Optical transparency may also be useful in sealingopto-electronic devices such as displays and photovoltaic devices, whichrely on light transmission. In embodiments, the hermetic barrier layershave an optical transparency characterized by an optical transmittanceof greater than 90% (e.g., greater than 90, 92, 94, 96 or 98%).

In one further example embodiment, sputter-deposited hermetic barrierlayers may be used to encapsulate a workpiece that contains a liquid ora gas. Such workpieces include dye-sensitized solar cells (DSSCs),electro-wetting displays, and electrophoretic displays. The disclosedhermetic barrier layers can substantially inhibit exposure of aworkpiece to air and/or moisture, which can advantageously preventundesired physical and/or chemical reactions such as oxidation,hydration, absorption or adsorption, sublimations, etc. as well as theattendant manifestations of such reactions, including spoilage,degradation, swelling, decreased functionality, etc.

Due to the hermeticity of the protective barrier layer, the lifetime ofa protected workpiece can be extended beyond that achievable usingconventional hermetic barrier layers. Other devices that can beprotected using the disclosed materials and methods include organicLEDs, fluorophores, alkali metal electrodes, transparent conductingoxides, and quantum dots.

Disclosed are sputtering targets and methods for forming sputteringtargets that comprise a low T_(g) glass material or precursor thereof,or an oxide of copper or tin. Sputtering processes using the foregoingtargets can be used to form self-passivating hermetic barrier layers.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “glass” includes examples having two or moresuch “glasses” unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, examples include from the one particular value and/or to theother particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It is also noted that recitations herein refer to a component being“configured” or “adapted to” function in a particular way. In thisrespect, such a component is “configured” or “adapted to” embody aparticular property, or function in a particular manner, where suchrecitations are structural recitations as opposed to recitations ofintended use. More specifically, the references herein to the manner inwhich a component is “configured” or “adapted to” denotes an existingphysical condition of the component and, as such, is to be taken as adefinite recitation of the structural characteristics of the component.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Sincemodifications, combinations, sub-combinations and variations of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and their equivalents.

We claim:
 1. A method of forming a hermetic barrier layer, comprising:providing a substrate and a sputtering target within a sputteringchamber, the sputtering target including a thermally conductive backingplate and a sputtering material comprising glass; maintaining thesubstrate at less than 23° C. during sputtering; and forming aself-passivating barrier layer comprising the sputtering material over asurface of the substrate at a deposition rate of about 10 Å/sec orgreater by actively cooling the sputtering target at less than about200° C. and sweeping the sputtering target with an ion beam at nearnormal incidence, wherein the self-passivating layer has a defect sizedistribution and defect density distribution less than a criticalself-passivation threshold such that defect diffusion paths are sealedthrough molar volume expansion of less than 10% of the sputteringmaterial when exposed to air or moisture.
 2. The method of claim 1,wherein the sputtering material more specifically comprises low T_(g)glass, a precursor of a low T_(g) glass, and/or an oxide of copper ortin.
 3. The method of claim 2, wherein the sputtering material isselected from the group consisting of phosphate glasses, borate glasses,tellurite glasses, and/or chalcogenide glasses.
 4. The method of claim3, wherein the sputtering material more specifically comprises amaterial selected from the group consisting of a tin phosphate, tinfluorophosphate and a tin fluoroborate.
 5. The method of claim 3,wherein composition of the sputtering material comprises: 20-100 mol %SnO; 0-50 mol % SnF₂; and 0-30 mol % P₂O₅ or B₂O₃.
 6. The method ofclaim 1, wherein, during the forming of the self-passivating barrierlayer, internal pressure of the chamber is maintained less than 10⁻³Torr.
 7. The method of claim 1, further comprising cooling the substrateto a temperature less than room temperature.
 8. A method of forming ahermetic barrier layer, comprising: providing a substrate and asputtering target within a sputtering chamber, the sputtering targetincluding a thermally conductive backing plate and a sputtering materialselected from at least one of a low Tg glass with a glass transitiontemperature of less than about 400° C. or a precursor of the low Tgglass; maintaining the substrate at less than 23° C. during sputtering;and forming a self-passivating barrier layer comprising the sputteringmaterial over a surface of the substrate at a deposition rate of about10 Å/sec or greater by actively cooling the sputtering target at lessthan about 200° C. and sweeping the sputtering target with an ion beamat near normal incidence, wherein the self-passivating layer has adefect size distribution and defect density distribution less than acritical self-passivation threshold such that defect diffusion paths aresealed through molar volume expansion.
 9. The method of claim 8, whereinthe molar volume expansion is of less than 15% of the sputteringmaterial when exposed to air or moisture.
 10. The method of claim 9,wherein the molar volume expansion is of less than 10% of the sputteringmaterial when exposed to air or moisture.
 11. The method of claim 8,wherein the molar volume expansion is from 1% to 15% of the sputteringmaterial when exposed to air or moisture.
 12. The method of claim 8,wherein, during the forming of the self-passivating barrier layer,internal pressure of the chamber is maintained less than 10⁻³ Torr. 13.The method of claim 8, wherein the sputtering material is selected fromthe group consisting of phosphate glasses, borate glasses, telluriteglasses, and/or chalcogenide glasses.
 14. The method of claim 13,wherein the sputtering material more specifically comprises a materialselected from the group consisting of a tin phosphate, tinfluorophosphate and a tin fluoroborate.
 15. A method of forming ahermetic barrier layer, comprising: providing a substrate and asputtering target within a sputtering chamber, the sputtering targetincluding a thermally conductive backing plate and a sputtering materialselected from at least one of a low Tg glass with a glass transitiontemperature of less than about 400° C. or a precursor of the low Tgglass; maintaining the substrate at less than 23° C. during sputtering;and forming a self-passivating barrier layer comprising the sputteringmaterial over a surface of the substrate by actively cooling thesputtering target at less than about 200° C. and sweeping the sputteringtarget with an ion beam at near normal incidence, wherein theself-passivating layer has a defect size distribution and defect densitydistribution less than a critical self-passivation threshold such thatdefect diffusion paths are sealed through molar volume expansion of lessthan 15% of the sputtering material when exposed to air or moisture. 16.The method of claim 15, wherein the sputtering material is selected fromthe group consisting of phosphate glasses, borate glasses, telluriteglasses, and/or chalcogenide glasses.
 17. The method of claim 16,wherein the sputtering material more specifically comprises a materialselected from the group consisting of a tin phosphate, tinfluorophosphate and a tin fluoroborate.
 18. The method of claim 15,wherein, during the forming of the self-passivating barrier layer,internal pressure of the chamber is maintained less than 10⁻³ Torr. 19.The method of claim 15, further comprising cooling the substrate to atemperature less than room temperature.
 20. The method of claim 15,wherein the molar volume expansion is of less than 10% of the sputteringmaterial when exposed to air or moisture.